Glacial rock flour (GRF) is a fine-grained silicate mineral formed below the Greenland Ice Sheet where the bedrock is abraded to a fine powder. GRF is transported by meltwater into fjords and coastal waters and its dissolution in seawater is part of the natural cycling of material between continents and the ocean. It is present in large sedimentary deposits along the coast of Greenland. However, due to the relatively small size distribution of GRF (d₅₀ ~ 2-5 µm) it has a relatively long residence time in the coastal surface layers and significant amounts reach the open ocean as suspended particulate material. As a silicate-rich material, also containing substantial amounts of micronutrients (e.g., iron and manganese), dissolution of GRF has the potential to both increase alkalinity and support phytoplankton growth. Therefore, it may be considered a source for large-scale marine CO₂ removal (mCDR). In this presentation we focus on its potential for supporting phytoplankton growth. We present results from incubation experiments in the field with natural phytoplankton communities and from climate-regulated laboratory experiments with a single-species phytoplankton culture. Field-incubations (6 days) with a subtropical phytoplankton community showed a significant increase in photosynthetic activity (Fv/Fm) in treatments with GRF. Similar field-experiments with natural communities from an Arctic fjord in Greenland, with a high natural background concentration of GRF, showed a modest or a neutral response to further addition of GRF. Long laboratory incubation experiments (3 weeks) with an Arctic green alga showed a significant increase in both growth rate and photosynthetic activity in treatments with GRF. The growth increased gradually with increasing concentrations of GRF until saturation was reached. This response was consistent with a simple model of trace-metal limited growth where micronutrients (e.g., iron) is biologically mobilized from GRF during the incubation period. These results show that substances in GRF, likely trace metals, can be biologically mobilized on timescales of days to weeks and thereby support growth of phytoplankton. Thus, GRF may be a source for large-scale mCDR due to its potential for increasing ocean productivity and strengthening the biological pump.

How to cite: Bendtsen, J., Daugbjerg, N., Vallentin Larsen, K., R. Vives, C., Dyrberg Dahms, R., Richardson, K., and Thorleif Rosing, M.: Glacial rock flour is a potential source for marine carbon dioxide removal by stimulating phytoplankton growth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13100, https://doi.org/10.5194/egusphere-egu24-13100, 2024.

Ocean Alkalinity Enhancement (OAE) allows for active removal of atmospheric CO₂, therefore is considered as one of the most promising Carbon Dioxide Removal (CDR) technologies. OAE could be obtained by discharging alkaline material in the wake of ships, however very little is known on potential negative effects on marine communities. We report here the first study focusing on the response of the entire pelagic microbial food web to the addition of calcium hydroxide in real oligotrophic conditions. In a mesocosm experiment performed at the CretaCosmos facility in Crete, Greece, in May-June 2023, we tested the response of the eastern Mediterranean oligotrophic waters to two different treatments of calcium hydroxide slurry addition (SL; High and Low concentrations, three replicate mesocosms each), while three more mesocosms served as Controls (no addition). Mesocosms, filled with natural coastal seawater, were treated with slurry on days 1, 3, 5, 7, 9, 11 to simulate the chronic disturbance, expected from repeated discharge of SL from ships; while the possible precipitation of carbonate crystals was assessed by putting a sediment trap at the bottom of each mesocosm. The carbonate-equilibrium and dissolution-kinetics were monitored by measuring temperature, solution-conductivity, and changes in pH. Photosynthetically-Active-Radiation and visible light were monitored by sensors in each mesocosm. Plankton productions (bacterial, viral, secondary) as well as community composition of all plankton groups from viruses to copepods were assessed by optical microscopy, flow cytometry and metagenomics; chlorophyll was also measured. Although an important alteration of pH was observed in the High lime addition, only heterotrophic bacteria production was found to be negatively affected and only in the second half of the experiment. The rest of the plankton groups presented different patterns and not a clear response to the lime addition. This first attempt to study the effect of lime addition on the complex pelagic food web will serve as a first step to an extensive testing needed before any application of ocean liming at a large scale.

How to cite: Pitta, P., Magiopoulos, I., Romano, F., Tsiola, A., Chantzaras, C., Gonzalez, J., Serret, P., Valsecchi, S., Varliero, S., Basso, D., Azzellino, A., Barbaccia, E., Traboni, C., Nocera, A. C., Courboules, J., and Tsapakis, M.: Ocean liming in the oligotrophic Eastern Mediterranean: impact on the planktonic microbial food web, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18538, https://doi.org/10.5194/egusphere-egu24-18538, 2024.

Our commitments to limit future global warming to below 2°C of the pre-industrial level are clashing with demonstrably insufficient present-day efforts to reduce CO₂ emissions. The development and implementation of Negative Emission Technologies (NETs), enabling a massive and fast CO₂ Removal (CDR), represent the most promising strategy to support an effective mitigation of the ongoing climate change within the next decade. Marine CDR (m-CDR) encompasses those technologies exploiting the ocean CO₂ storage potential, and there is an increasing number of international initiatives aimed at assessing their possible impact on marine communities. Ocean liming consists in spreading alkaline substances, such as calcium hydroxide (slaked lime), on surface ocean waters. Slaked lime reacts with surface marine waters by triggering m-CDR from the atmosphere and ocean alkalinity enhancement, thus contrasting ocean acidification. Although ocean liming has already been assessed as chemically effective and economically sustainable, the scientific scrutiny of its potential impacts on the ocean biota has just started. Previous laboratory and mesocosm experiments showed the occurrence of transient pH peaks, which may impact the pelagic ecosystem by selecting less sensitive species, and runaway precipitation of aragonite particles after concentrated and repeated liming, which reduces the efficiency of CDR and negatively affects both plankton and benthos by mechanical clogging and choking. Nutrients exert a major control on primary producers, and higher salinity may affect the carbonate kinetics by facilitating CaCO₃ precipitation. For these reasons, two mesocosm experiments of liming, funded by the EU2020 project AQUACOSM-plus and the OACIS-initiative of the Fondation-Prince-Albert-II-de-Monaco, were conducted with a comparable experimental design at the CIM-ECIMAT (University of Vigo) and CRETACOSMOS (Hellenic Centre for Marine Research) facilities. The aim was to contrasting the response to ocean liming of the eutrophic Ría de Vigo upwelling system (eastern Atlantic) and the eastern Mediterranean ultraoligotrophic and more saline setting. The preliminary results of repeated additions of slaked lime in the two different types of marine coastal waters, and the response of calcareous nannoplankton and benthic calcareous red algae (coralline algae) to the observed chemical changes are presented here, suggesting the need to optimize and modulate the mCDR techniques, in order to meet the specific geochemical and biological characteristics of the different water bodies.

How to cite: Basso, D., Bazzicalupo, P., Varliero, S., González, J., Serret, P., Pitta, P., Alcaráz, P., Penín, A., Macchi, P., Raos, G., Barbaccia, E., Magiopoulos, I., Tsiola, A., Romano, F., Valsecchi, S., and Azzellino, A.: Ocean liming in eutrophic vs. ultraoligotrophic environments and the response of algal calcifiers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13685, https://doi.org/10.5194/egusphere-egu24-13685, 2024.

Ocean alkalinity enhancement (OAE) has been proposed as a strategy to sequester carbon dioxide (CO₂) from the atmosphere by adding alkaline substances to seawater. In addition to alkalinity, various dissolution products could be released under OAE, depending on the choice of alkali mineral used. These products such as silicate and changes in carbonate chemistry can impact the competitive fitness of phytoplankton species, which could directly or indirectly affect the compositions of the phytoplankton community. Currently, there are knowledge gaps pertaining to the potential ecological impacts of alkalinisation on natural phytoplankton communities, which hamper a comprehensive evaluation of OAE for its large-scale implementation.

To address these gaps, we carried out an in situ mesocosm experiment examining the response of a natural plankton community over 53 days in the temperate mesotrophic waters of the Raunefjord south of Bergen, Norway to two alkali mineral applications. We simulated two mineral types, a calcium-based (quicklime) and silicate-based (olivine) alkalinisation in a non-equilibrated approach. NaOH was used in both mineral treatments to establish a gradient of six alkalinity levels ranging from ambient (~2400 µmol kg^-1) to ~3000 µmol kg^-1 in steps of 150 µmol kg^-1. Silicate-based and calcium-based alkalinisation were simulated through the addition of MgCl₂ and CaCl₂, respectively. Additionally, the treatment simulating olivine-based OAE received 70 µmol L^-1 of Si(OH)₄. Since phytoplankton was nutrient limited from the onset of the experiment, nutrients (nitrate, phosphate) were added halfway through the study to allow for an explicit detection of responses.

Here we report on the responses of the phytoplankton community to the simulated OAE scenarios. Our results indicate that phytoplankton abundances remained largely unaffected across the alkalinity gradient and between mineral types during the oligotrophic phase of the experiment. However, significant differences in the phytoplankton community response were observed post nutrient addition. Here, coccolithophores exhibited a negative response to increasing alkalinity in the silicate-based treatment, whereas the correlation was relatively weak in the calcium-based treatment. We attribute these responses, in part, to changes in carbonate chemistry such as low pCO₂, which may limit coccolithophore growth and the out-competition by diatoms favoured by added silicate.

Overall, our findings suggest minimal risks associated with OAE under oligotrophic conditions over a 20-day period. However, the potential for species-specific negative impacts of increasing alkalinity should be carefully considered under high nutrient availability. These results represent a crucial first step towards understanding the ecological responses of phytoplankton communities, helping to define the safe operating space in non-equilibrated OAE implementations. 

How to cite: Xin, X., Kittu, L. R., Ortiz Cortes, J., Groen, A. W., and Riebesell, U.: Responses of phytoplankton community to silicate-based and calcium-based ocean alkalinity enhancement, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-944, https://doi.org/10.5194/egusphere-egu24-944, 2024.

Ocean alkalinity enhancement (OAE) can help mitigate climate change impacts by increasing the carbon storage capacity of the ocean. The technique involves addition of alkaline substances to the seawater to accelerate the natural rock weathering process. However, this will lead to sudden seawater chemistry changes, such as increased pH that might directly and/or indirectly (through trophic pathways) affect zooplankton, an important trophic link, by altering its metabolic state and community composition. In addition, varying dilution times of alkaline substances might impact organisms differently. To date, the possible influences of OAE on zooplankton communities are largely unexplored. To bridge the knowledge gap, we conducted mesocosm and laboratory experiments in simulated non-equilibrated, calcium-based (Ca(OH)₂) OAE setups. An incrementally enhanced alkalinity gradient from 0 to 1250 µmol kg^-1 in steps of 250 µmol kg^-1 was used in all experiments. The wide-ranging enhanced total alkalinity (∆TA) was selected to assess the safety threshold. In addition, we compared immediate versus delayed dilution scenarios in our mesocosm study, where each scenario ended up with the same ∆TA gradient after mixing. We examined the multitrophic community response by monitoring twelve mesocosms for 39 days including the natural spring bloom community of Helgoland roads waters in the North Sea. Subsequently, the direct effect of alkalinity enhancement on the physiology (i.e., respiration and grazing) of Temora longicornis (predominant copepod in the mesocosms) was evaluated in the laboratory. The species-specific bottom-up effect was examined by culturing Rhodomonas salina in aforementioned ∆TA gradient and feeding them to the T. longicornis. We observed relatively lower zooplankton abundance, and growth rate in mesocosms with ∆TA1000 and 1250 µmol kg^-1, which might be a bottom-up effect. In our lab experiments, though, we observed a negative impact on R. salina growth rate and nutritional quality from ∆TA750 µmol kg^-1, we did not detect any substantial direct or indirect impact on the physiological performance of T. longicornis. Overall, our laboratory study provided a preliminary understanding of the direct and indirect effects of OAE on a key copepod species, and the mesocosm study gave insight into the zooplankton community response.

How to cite: Bhaumik, A., Henning, M., Faucher, G., Kittu, L., Schneider, J., Meunier, C. L., Riebesell, U., and Boersma, M.: Assessing the impact of Ocean Alkalinity Enhancement on the zooplankton community, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-300, https://doi.org/10.5194/egusphere-egu24-300, 2024.

Marine-based electrochemical Carbon Dioxide Removal (mCDR) is a rapidly evolving subject area. Technology is being developed that facilitates atmospheric CO₂ removal by extracting Dissolved Inorganic Carbon (DIC) from seawater. Decarbonated seawater, that also has a high pH, is released into the marine environment, facilitating the drawdown of atmospheric CO­₂ into the surface ocean. Chemical perturbations also include low levels of carbon dioxide (CO₂) and bicarbonate (HCO₃^-),and increased levels of carbonate (CO₃^2-). There is no literature that investigates the impact of low carbon seawater with elevated pH on marine ecosystems. Understanding and cataloguing the effect of mCDR is fundamental for: i) determining potential impact on vulnerable systems; ii) supporting the development of any necessary mitigating actions; iii) confirming overall mCDR effectiveness; and iv) engaging the public and harnessing their support.

This work presents results from laboratory experiments that examine the physiological response of keystone organisms to decarbonated and high pH seawater. Decarbonated high pH seawater released into the environment will be diluted by mixing with ambient seawater, such that the chemical perturbations become less extreme with distance from source. Intertidal mussels (Mytilus edulis) are a keystone species that utilize DIC for major cellular functions and have poor acid-base balance. Mussels were exposed to three different dilutions of decarbonated high pH seawater (generating pH values of approximately, 10, 9.2 and 8.7). Mortality, oxygen consumption rate and filtering rate were measured after short-term (48 hr) exposure and then 48 hrs after returning to ambient seawater. Initial experiments indicate that undiluted decarbonated high pH seawater has a significant short-term impact on the physiological response of Mytilus edulis, but the species shows signs of recovery following a week in ambient seawater. Data from these and other experiments will be used to generate a risk gradient that illustrates how physiological response(s) change with dilution of low carbon, high pH seawater discharge.

How to cite: Hooper, G., Findlay, H., Bell, T., and Halloran, P.: Impact of decarbonated and high pH seawater on the physiology of intertidal mussels, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-657, https://doi.org/10.5194/egusphere-egu24-657, 2024.

The natural dissolution of calcium- or silicate-based rock minerals in the ocean increases the alkalinity and enhances the uptake of atmospheric CO₂. Deliberate large-scale addition of such minerals to the surface ocean has been proposed as a promising method to drive negative CO₂ emissions through ocean alkalinity enhancement (OAE), mitigating climate change. However, the environmental safe and sustainable implementation of OAE requires a comprehensive understanding of the potential ecological implications of this marine-based Carbon Dioxide Removal technology. In contributing to this understanding, a 39-day mesocosm experiment was conducted in the temperate-eutrophic waters of the German North Sea off Helgoland, during the spring of 2023. The primary objective was to examine how the intensity of alkalinity and the duration of alkalinity exposure before dilution in a calcium-based non-equilibrated OAE application (elevated pH) affects the pelagic ecology and biogeochemistry during a phytoplankton spring bloom. We simulated alkalinisation via calcium hydroxide through the addition of calcium chloride and sodium hydroxide in total alkalinity (∆TA) increments of 250 µmol kg^-1 (∆TA = 0, 250, 500, 750, 1000, 1250 µmol kg^-1) in one set of six mesocosms (each with a volume of 6 m³). This treatment intended to represent the successful dilution of OAE application through ship-deployment. A second set of six mesocosms was used to simulate a delayed dilution of alkalised waters from a point source. For this, the top layer of these mesocosms was manipulated with twice the amount of TA and mixed with the untreated bottom layer after 48 hours, ultimately leading to the same ∆TA gradient as the immediate dilution treatment. Here, we report on the influence of OAE on phytoplankton bloom dynamics and particulate matter stoichiometry, which are key characteristics of marine ecosystems and carbon cycling. The first results indicate a delay in phytoplankton bloom timing with increasing alkalinity and pH, with no discernible impact of dilution type. Surprisingly, significant differences in Chlorophyll a dynamics at the lowest ∆TA level of 250 were observed in both dilution types. Furthermore, peak concentrations of particulate organic carbon (POC) exhibit a significant decrease with increasing ∆TA and pH in the delayed dilution treatment, particularly evident in the two highest ∆TA treatments. Conversely, the immediate dilution treatment displays a positive trend in POC with increasing ∆TA and pH, indicating the influence of alkalinity intensity and duration of alkalinity exposure before dilution on bulk POC build up by phytoplankton. Given that changes in phytoplankton bloom dynamics and particulate organic matter can alter the ocean’s CO₂ uptake and sequestration potential, our results address significant knowledge gaps to determine an ecologically safe operating space for OAE implementation under nutrient rich conditions.

How to cite: Tammen, J., Kittu, L. R., Faucher, G., Schulz, K. G., and Riebesell, U.: Assessing the Influence of OAE on Particulate Matter Stoichiometry in the North Sea – Insights from a Mesocosm Study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-436, https://doi.org/10.5194/egusphere-egu24-436, 2024.

Throughout the past decades, the rise of atmospheric carbon dioxide (CO₂) levels has been one of the most important global issues. Higher CO₂ concentrations contribute to a strengthened greenhouse effect, resulting in elevated temperatures and a severe acidification of the oceans.

Recently, the Intergovernmental Panel on Climate Change (IPCC) highlighted the need to develop CO₂ removal approaches, as an essential support to mitigate the ongoing climate change. To this purpose, Negative Emission Technologies (NETs) are capable of extracting CO₂ from the atmosphere, keeping it stored in geological reservoirs for long periods.

Among NETs, Limenet s.r.l. is proposing a pH equilibrated Ocean Alkalinity Enhancement (OAE) process which involves the permanent storage of carbon dioxide in seawater in the form of bicarbonates using calcium hydroxide and releasing a carbon enriched solution at the same pH of natural seawater. The life cycle assessment conducted on this process demonstrated that the advantages of CO₂ capture and storage outweigh the greenhouse gas emissions produced by the entire process.

Although this technology is economically promising and the chemical analysis has shown that CO₂ stored in the form of bicarbonates in the seawater is quite stable, it’s necessary to assess any possible impact of the pH equilibrated OAE on marine organisms. In light of this, the aims of this project are:

1) To assess the short-term response of the biota after the treatment.

2) To study the effects of a prolonged exposure to the treated water on planktonic and benthic communities through mesocosms experimentation.

All experiments are conducted in the Gulf of La Spezia (North-West Italy).

How to cite: Calvi, D., Groppelli, S., Campo, F., Comazzi, F., Basso, D., and Cappello, S.: First assessment of the impact of pH equilibrated Ocean Alkanlinity Enhancement technology on marine biota in the Gulf of La Spezia (north-west Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16102, https://doi.org/10.5194/egusphere-egu24-16102, 2024.

Bringing carbon dioxide (CO₂) removal approaches from the laboratory to the industrial scale in the following years is imperative to reaching Net Zero goals. Ocean Alkalinity Enhancement (OAE) is a promising approach that introduces alkalinity into surface waters, slightly modifying the carbonate equilibrium and thus increasing the seawater’s CO₂ removal capacity. The co-location of OAE with existing coastal industries (e.g., desalination plants) is a way to accelerate their deployment.

Recognizing the importance of balancing climate mitigation strategies with environmental stewardship, our study focuses on the outcomes of comprehensive ecotoxicity tests conducted to discern the effects of return flows from desalination plants (concentrated seawater or brine) enriched with alkalinity (carbonates). To simulate real conditions, we investigated three scenarios: (i) brine alone, (ii) alkalinity alone, and (iii) brine with added alkalinity. The ecotoxicity tests were designed to capture the responses of key marine organisms across various trophic levels. Bacterial assays illuminated the microbial community's sensitivity to different scenarios, while diatom assessments provided insights into primary producers' adaptive capacity. Copepod and crustacean tests explored the cascading effects on higher trophic levels, elucidating potential ramifications for marine food webs.

Our findings shed light on the intricate interplay between alkalinity-enhanced brines and local ecosystems, providing valuable insights into potential stressors and their implications for marine biota. The ecotoxicity results at different dilutions are combined with the knowledge of mixing zones for ocean outfall technologies, putting forward the environmental impact at different distances from the alkalinity addition.

By addressing the specific challenges posed by integrating OAE with desalination, this research contributes to the ongoing dialogue on responsible and sustainable climate mitigation strategies. It offers a nuanced understanding of the potential trade-offs and synergies between addressing climate change and preserving or benefitting local marine environments. At a time when environmental testing at scale is needed, this study assesses the potential risks of such research. Ultimately, this work facilitates dialogue among OAE companies, desalination experts, policymakers, and organisms for environmental control and protection.  

How to cite: Buceta, J. and Sdez, N.: Integration of Ocean Alkalinity Enhancement Processes with Coastal Industries: Ecotoxicity Assessment for Local Environmental Impacts, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2483, https://doi.org/10.5194/egusphere-egu24-2483, 2024.

Climate change poses a critical threat to global ecosystems and human well-being, necessitating innovative solutions for carbon dioxide (CO₂) mitigation. This study employs the UVic-ESCM, an Earth system model of intermediate complexity, to investigate the potential and possible side effects of two marine-based CO₂ removal techniques, namely ocean alkalinity enhancement and macroalgae farming and sinking. Additionally, simulations from Bern3D-LPX for ocean alkalinity enhancement provide a model comparison.

Focusing on not only warming but also acidification and deoxygenation, the research aims to compare the theoretical deployment of these techniques in an emission-driven overshoot scenario (SSP5-3.4).To encompass uncertainty due to model parameters, we analyzed a perturbed parameter ensemble constrained by observations. Preliminary findings indicate that both techniques show promise in mitigating atmospheric CO₂ concentrations, with variations in their effects on climate and oceanic conditions. Both techniques show small cooling potential despite the large-scale theoretical deployment. While they do not provide an alternative to emission reductions, they could be beneficial in combating other human-induced stressors in the marine ecosystem. Both techniques demonstrate a potential to counteract ocean acidification, but we find that macroalgae farming and sinking contributes to localized deoxygenation.

This study contributes to the ongoing discourse on so-called ‘nature-based’ solutions for climate change mitigation by offering a nuanced evaluation of the theoretical upper potential in multiple mitigation dimensions as well as side effects of ocean alkalinity enhancement and macroalgae farming and sinking. The outcomes aim to inform future research directions and decision-making processes towards the development of effective and ecologically sustainable carbon dioxide removal strategies.

How to cite: Tran, G., Jeltsch-Thömmes, A., Keller, D., Oschlies, A., and Joos, F.: Assessing the Mitigation Potential and Ecological Impacts of Carbon Dioxide Removal Technologies on Ocean Ecosystems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15549, https://doi.org/10.5194/egusphere-egu24-15549, 2024.

Ocean alkalinity enhancement is one approach being considered to contribute to marine Carbon Dioxide Removal techniques. It relies on the addition of alkalinity to the marine environment, either under the form of crushed-rock feedstocks or under a dissolved form. This conducts to the decrease of seawater partial pressure of CO₂ (pCO₂) allowing seawater, by equilibrium with air, to absorb more CO₂ from the atmosphere.

In order to determine the amount of CO₂ removed from the atmosphere, a monitoring, reporting, and verification (MRV) system of marine carbon dioxide removal needs to be designed. The evaluation of ocean alkalinity enhancement will depend on the monitoring of measurable variables of the marine carbonate system, as well as on numerical simulations. In this context, acquiring accurate and robust data of seawater total alkalinity is highly relevant in order to quantify the background state alkalinity and to check that alkalinity has efficiently been added.

In that goal, the application of the three pillars of metrology: metrological traceability, measurement procedure harmonization and validation, and uncertainty estimation, are required. However, up to date, the total alkalinity measurement method lacks of a rigorously established uncertainty budget.

The establishment of measurement results uncertainty can be realised by the mean of two approaches. The “bottom-up” approach is the most rigorous way to thoroughly establish an uncertainty budget. It relies on the identification and quantification of each source of uncertainty involved at every step of the measurement process, as described by the Guide to the expression of Uncertainty in Measurements. The “top-down” approach relies on an experimental assessment of the uncertainty, from repeatability, reproducibility and trueness estimates.

The presentation will focus on the evaluation of the uncertainty of seawater total alkalinity measurement results using the two approaches aforementioned. The sources of uncertainty originating from the potentiometric titration measurement method, and the mathematical model used for data treatment, will be presented and quantified. This study will also allow identifying which sources have the major contribution to the overall uncertainty budget, and thus the ones we should focus on to lower the uncertainty. The estimation of the uncertainty with the “top-down” approach is determined from an inter-laboratory comparison involving five laboratories, conducted on reference solutions. The developed artificial and natural seawater reference solutions, as well as the results of the inter-laboratory comparison, will be presented. Comparison, advantages and limitations of the two uncertainty estimation methods will be discussed. Finally, the level of uncertainty estimated will be discussed in the frame of MRV system in supporting the evaluation of ocean alkalinity enhancement.

How to cite: Capitaine, G., Fisicaro, P., and Wagener, T.: Quantification of seawater total alkalinity measurement uncertainty to support the evaluation of ocean alkalinity enhancement, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7748, https://doi.org/10.5194/egusphere-egu24-7748, 2024.

Ocean Alkalinity Enhancement (OAE) emerges as a promising strategy capable of sequestering several gigatons of CO₂ annually from the atmosphere and store it in the ocean for extended periods (>1000 years). To achieve this objective, artificial alkalinity is introduced into the surface ocean through alkaline solutions or the spontaneous dissolution of alkaline solids. When contemplating alkaline solids for OAE, a primary challenge lies in generating substantial quantities of fine grained (<10 µm), soluble solids at low energy and cost. The hydration of carbonates presents a potentially less energy-intensive method, yielding products that exhibit favorable thermodynamics leading to their spontaneous dissolution in seawater¹.

We investigated the stability and dissolution kinetics of two hydrous carbonates, Ikaite (CaCO₃·6H₂O) and water-bearing amorphous calcium carbonate (CaCO₃.nH₂O), labelled hereafter as w-ACC. Both phases can be created from dissolving limestone at high CO₂ pressures. An engineering concept using a CO₂ pressure swing in a reactor has been recently published¹. Once created,  the hydrous carbonate phases are unstable at temperatures higher than the formation temperature and transform to anhydrous polymorphs after a certain period of time. Thus, we have investigated the temporal stability of either phase at different temperatures in order to contribute to their life cycle assessment. Moreover, the transformation of ikaite and w-ACC to an anhydrous polymorph obliterates the effect of releasing alkalinity during spontaneous dissolution, which needs to be avoided. Our results show that at room temperature both phases dehydrate within hours when stored as wet powders after simple filtration. However, their stability extends to days when the physical adsorbed water is removed e.g. by rinsing with ethanol. A quantitative estimate of the kinetic rate of the hydrous-to-anhydrous phase transformation is currently being analyzed by Raman spectroscopy .

Our results also indicate that w-ACC has a higher dissolution rate than ikaite in seawater due to its higher specific surface area (>90m²/g). However, the efficiency of both hydrated carbonates in releasing alkalinity will be further analyzed to elucidate the effect of particle coagulation, particle sinking, and secondary precipitation phenomena. Nonetheless, our pilot results demonstrate that both ikaite and w-ACC are promising candidates for OAE, considering their potential in augmenting ocean alkalinity and CO₂ sequestration.

1             Renforth, P., Baltruschat, S., Peterson, K., Mihailova, B. D. & Hartmann, J. Using ikaite and other hydrated carbonate minerals to increase ocean alkalinity for carbon dioxide removal and environmental remediation. Joule 6, 2674-2679 (2022). https://doi.org/10.1016/j.joule.2022.11.001

How to cite: Baltruschat, S., Bastianini, L., Millar, R., Mihailova, B., Foteinis, S., Thoutam, P., Lu, X., Hartmann, J., Yang, A., and Renforth, P.: Considering hydrous carbonates for ocean alkalinity enhancement, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12695, https://doi.org/10.5194/egusphere-egu24-12695, 2024.

Ocean Alkalinity Enhancement (OAE) is a proposed mechanism of atmospheric CO₂ removal, or negative emission technology. The addition of mineral particles to natural waters is one potential method of achieving OAE, yet there are substantial uncertainties concerning the efficiency of this process in terms of total alkalinity (TA) generation under natural conditions. Laboratory incubations are generally conducted under standardized conditions with filtered, deionized or sterile water which, whilst necessary to conduct reproducible mechanistic studies, is clearly not representative of most natural waters in which OAE might be deployed. In order to assess how variation in natural water properties affects the dissolution of minerals proposed as OAE agents, here we test the conversion of Mg(OH)₂ to total alkalinity (TA) in river water, estuarine water, coastal seawater and offshore seawater. We found that when added as milk of magnesia, a 10 g/L (final concentration) Mg(OH)₂ dose was efficiently converted to TA (>95% efficiency) in river water with low initial TA (mean TA = 239.44), river water with medium initial TA (mean TA = 1636.13), high salinity estuarine water (salinity 27), low salinity estuarine water (salinity 5.3), and seawater. However, when Mg(OH)₂ was applied to high TA river water as a single dose (10 mg/L), the TA increase was only 60% of the calculated addition. The effect of multiple small doses (2.5 mg/L) was also tested, with no significant difference in the TA conversion in most cases. Dry additions of Mg(OH)₂, rather than pre-mixed suspensions of milk of magnesia, were found to be inefficient TA sources, sometimes leading to negative TA changes- especially when using small incubation bottles (2 L). Overall it was demonstrated that Mg(OH)₂ can be used as an efficient TA source in most natural waters for final doses in the range 2.5-10 mg/L.

How to cite: Wang, X., Shi, J., Huang, X., Bai, R., Qi, Y., Niu, R., and Hopwood, M.: Evaluating Mg(OH)2 as an ocean alkalinity agent in tropical river, estuary and saline water, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-812, https://doi.org/10.5194/egusphere-egu24-812, 2024.

The deployment of carbon dioxide storage is a challenge that must be overcome in order to reach the net zero emissions objective. Ocean alkalinity enhancement (OEA) is a promising method for removing carbon dioxide from the atmosphere and storing it pemanently in seawater as bicarbonates, with the co-benefit of counteracting ocean acidification. The challenge for future applications is ensuring a stable storage, avoiding adverse side effects on the environment or phenomena that can reduce efficiency, such as degassing of carbon dioxide and precipitation of alkaline minerals.

The work presented in this study investigates the stability of the carbonate system of seawater, after adding alkalinity by two different pathways. One is the simple addition of NaHCO₃ to artificial seawater, on a laboratory scale. The other is a test on a more complex system, consisting of a pilot plant that uses natural seawater, CO₂ and calcium hydroxide and produces a carbon-enriched solution at the same pH of natural seawater. We suggest safe levels for the increase of alkalinity and considering the dilution of the solution with seawater in natural environments. The results represent important steps towards the achievement of safe and efficient ocean-based carbon storage and OAE.

How to cite: Varliero, S., Comazzi, F., Campo, F. P., Cappello, S., Cappello, G., Caserini, S., Macchi, P., and Raos, G.: Experimental studies on the stability of bicarbonate-enriched seawater solutions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15887, https://doi.org/10.5194/egusphere-egu24-15887, 2024.

The marine carbon dioxide removal (mCDR) industry is undergoing rapid growth, with many stakeholders deploying mCDR pilot projects. In order to establish a scientific basis for environmental safety and carbon accounting in mCDR, there is a need for rigorous, transparent and scientifically robust monitoring, reporting, and verification (MRV) protocols. These protocols seek to ensure responsible scaling of mCDR projects that have demonstrable net-negative atmospheric impacts. One of the main MRV challenges facing the mCDR pathway of Ocean Alkalinity Enhancement (OAE) is that direct measurements in the marine environment can be difficult to obtain. Here, we present the highlights of the first version of the Isometric OAE MRV protocol and focus on the process for interdisciplinary collaboration that informed decision-making and iteration towards the current version. We focus specifically on the development of guidelines for quantifying additionality, durability and uncertainty in the open system OAE pathway, and elaborate on our modeling requirements and benchmarks, as well as guidance on how models should be validated with environmental data. Ultimately, we aim to receive feedback on the protocol and our approach in order to apply this method to new versions and additional protocols and modules across mCDR and other CDR pathways. 

How to cite: Gill, S., He, J., Yin, J., Sutherland, K., Lambert, J., and Hacker, N.: Interdisciplinary collaboration to develop a robust and implementable monitoring, reporting, and verification (MRV) protocol for Ocean Alkalinity Enhancement (OAE), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10853, https://doi.org/10.5194/egusphere-egu24-10853, 2024.

Surface ocean alkalinity enhancement (OAE), through the release of alkaline materials, is an emerging carbon dioxide removal (CDR) technology that could increase the storage of anthropogenic carbon in the ocean. Although essential, evaluating the effects of alkalinity additions on the carbonate system and ultimately on air-sea CO₂ fluxes is not straight forward. Observations, even with autonomous platforms, are inherently sparse and limited, and therefore cannot provide a comprehensive quantification of the effects of OAE. Numerical models are important complementary tools. They can help guide fieldwork design, provide forecasts of the ocean state, and simulate the effects of alkalinity additions on the seawater carbonate system. Here we describe a coupled physical-biogeochemical implementation of ROMS in a nested grid configuration that reaches a very high spatial resolution in Bedford Basin (51m), a coastal fjord in eastern Canada that is chosen as a test site for OAE. The biogeochemical model simulates oxygen dynamics and the carbonate system, including air-sea gas exchange. We present a multi-year hindcast validated against the long-term weekly time series available at the Compass Buoy station in the centre of the Basin as well as recent simulations carried out during alkalinity addition trials. We will discuss the model’s capabilities with respect to OAE and the challenges ahead.

How to cite: Laurent, A., Wang, B., Pei, Q., Ohashi, K., Sheng, J., Garcia Larez, E., Fradette, C., Rakshit, S., Atamanchuk, D., Azetsu-Scott, K., Algar, C., Wallace, D., Burt, W., and Fennel, K.: A high-resolution nested model to study the effects of alkalinity additions in a mid-latitude coastal fjord, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6536, https://doi.org/10.5194/egusphere-egu24-6536, 2024.

Carbon Dioxide Removal (CDR) technologies are imperative for achieving net zero emissions, a crucial feat to meet the 2º target set in the Paris Agreement. Ocean Alkalinity Enhancement (OAE) is a marine CDR technology that consists of increasing the Total Alkalinity (TA) of the ocean by depositing alkaline minerals to ocean surface waters. The increase in TA reduces the sea surface partial pressure of CO₂ (pCO₂), thereby enhancing oceanic CO₂ uptake or reducing oceanic CO₂ outgassing. Despite the potential of OAE to reduce atmospheric CO₂ concentrations, the realistic implementation of OAE faces substantial impediments, including logistical feasibility and the lack of international ocean governance for its deployment in open waters. To address these obstacles and incentivize the development of a policy framework for OAE, we set forward optimal conditions that maximize the efficiency of OAE in the North Pacific Ocean, leveraging natural climatic variability induced by the Pacific Decadal Oscillation (PDO). The addition of TA at high Dissolved Inorganic Carbon (DIC) concentrations has the potential to induce a stronger decrease in pCO₂ than at lower DIC concentrations. Therefore, natural temporal increases in surface DIC concentrations could potentially predispose the system for enhanced OAE efficiency. The PDO induces multi-decadal variations in the carbonate system, with the potential to influence the spatiotemporal variability in OAE efficiency. PDO phases have been shown to be predictable up to a decade ahead, thereby providing a practical indication for logistical planning of OAE deployment. We analyze the influence of the PDO on OAE efficiency in the North Pacific Ocean through four Earth System Model simulations under a high emission scenario (RCP8.5) spanning from 2020 to 2100. Using theoretical CO₂ uptake efficiencies, as defined by Tyka et al. (2022) and Renforth and Henderson (2017), we describe how PDO states modulate variability in uptake efficiency via their control on DIC and TA concentrations. Subsequently, we analyze the realized uptake efficiency by contrasting oceanic air-water CO₂ fluxes (FCO₂) in simulations with continuous and homogenous global OAE deployment against simulations without CDR intervention per unit of added TA. Early results show regional differences in OAE efficiency rates during different PDO phases. During positive PDO phases, theoretical CO₂ uptake efficiencies decrease in the Northeast Pacific while increasing in the central Western Pacific, corresponding to respectively lower and higher DIC concentrations. The inverse responses are observed during negative PDO phases. We discern differences between theoretical and realized CO₂ uptake efficiencies, indicating the role of additional influential variables. Our study provides new insights into the impact of the PDO on OAE efficiency and the potential to optimize CDR strategies by aligning them with natural climatic variations.

How to cite: van Langen Rosón, A., C. Franco, A., Bernardello, R., Schwinger, J., Keller, D., and Wey, H.-W.: Ocean Alkalinity Enhancement efficiency in the North Pacific under influence of the Pacific Decadal Oscillation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12290, https://doi.org/10.5194/egusphere-egu24-12290, 2024.

To limit global warming to below 2°C by 2100, carbon dioxide removal (CDR) from the atmosphere will be necessary. Ocean alkalinity enhancement (OAE) is a promising approach to achieving CDR at a large scale. However, OAE deployments are subject to slow or incomplete air-sea CO₂ exchange, reducing the efficiency of carbon removal, defined as the excess CO₂ uptake per mol of alkalinity addition. We used a coupled ocean circulation-biogeochemistry model to generate the first global, time-resolved map of OAE efficiency across four different seasons and investigated its controlling factors. An ensemble of alkalinity pulse injections in the global ocean were simulated with the global 1-degree ocean component of the Community Earth System Model version 2 (CESM2). Alkalinity was added to the surface ocean for 1 month in a total of 690 patches and in 4 different seasons of a year. Each simulation was run for 15 years for each patch and season to compute OAE efficiency, residence time of excess alkalinity retained in the mixed layer, and CO₂ re-equilibration timescales - all referenced to the geographic location of the induced perturbation. OAE efficiency showed large spatial and seasonal variations. The highest seasonal mean OAE efficiency achieved after 15 years, ranging from 0.7 to 0.9, were found in the subpolar oceans, the semi-closed regions, such as the Gulf of St. Lawrence and the North Sea, as well as the coastal zones along the Pacific and South Atlantic. The lowest seasonal mean, ranging from 0.3 to 0.5, was found in the high latitudes Atlantic and Southern Ocean where deep water forms. The intermediate values, ranging from 0.5 to 0.7, were found predominantly in the subtropical gyres, as well as western and eastern boundary currents. Seasonally, higher maximum OAE efficiency could generally be achieved when alkalinity is released in the summer rather than in winter. Accurate understanding of the CO₂ response curves, as provided by our maps, is critical for choosing suitable OAE deployment sites and is central to the MRV (Measurement, Reporting & Verification) challenge faced by all marine CDR methods. 

How to cite: Zhou, M., Tyka, M. D., Bachman, S., Yankovsky, E., Karspeck, A. R., Ho, D. T., and Long, M. C.: A global efficiency map of ocean alkalinity enhancement (OAE) for CO2 removal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1429, https://doi.org/10.5194/egusphere-egu24-1429, 2024.

Negative emission technologies (NETs) are an integral part of most climate change mitigation scenarios limiting global warming to 1.5 °C above pre-industrial levels. Several different NETs have been proposed, including ocean alkalinization and direct CO₂ removal which have been considered as methods with high carbon removal potential. In ocean alkalinization partial pressure of CO₂ sea surface is reduced by spreading alkaline material and in direct removal of CO₂ it is extracted from sea water and transported to permanent reservoir. To date, most studies on ocean-based NETs with Earth System Models have been
 based on idealized scenarios where atmospheric carbon is either simply removed by prescribed amount or some NET is deployed at magnitudes that would be extremely challenging to reach if any economic, technical, or political constraints were considered.

In this work, we present Earth System Model simulations using a more realistic global deployment scenario for ocean alkalinization with CaO dispersed at ocean surface in the exclusive economic zones of US, Europe, and China. The dispersion scenario is based on current excess capacities in the lime and cement industries in these three regions, and high-end projections on how they could evolve until 2100. We use the high-overshoot SSP5-3.4-OS as the socioeconomic background scenario. We simulate the deployment scenarios with several Earth System Models. We will show results from simulations with alkalinity enhancement deployment initiated in 2030 and 2040. Furthermore, we compare these results with simulations of direct removal of CO₂. Here, the direct removal is calculated from the added alkalinity using approximation for CO₂ uptake factor using the relation between alkalinity and dissolved inorganic carbon.

The results show that the CO₂ is being removed from the atmosphere to oceans after the alkalinity deployment. Compared to the control simulation the global CO₂ concentration is reduced by about 7 ppm in the deployment scenario starting in 2030 and about 4 ppm in the deployment scenario starting 2040 by end of the century. For real life deployment the efficacy and detectability of the alkalinity enhancement is a major concern. We will show that the temperature change in the earlier deployment scenario (higher removal potential) cannot be distinguished from the annual variability illustrating the problem in detectability. Furthermore, the simulations show the deployment must be constrained in regions with low oceanic transport to inhibit the precipitation of CaCO₃ to retain the CO₂ removal potential.

Using a more realistic scenario for ocean alkalinization we can give a more realistic assessment of its climate effects and explore new research questions such as detectability of local changes in pH or carbon fluxes with slowly increasing deployment rates. In the realistic deployment scenario, ocean alkalinization decreases the CO₂ concentration but does not produce a large signal in the temperature. Therefore, this method can be seen as having potential but its role in removing carbon from the atmosphere is limited, according to these scenarios. Furthermore, the wider effects on the Earth system still require more analysis.

How to cite: Bergman, T., Bourgeois, T., Schwinger, J., Foteinis, S., Renforth, P., Seifert, M., Hauck, J., Keller, D., Muri, H., and Partanen, A.-I.: CO2 Removal Potential of Two Ocean-based NETs in Earth System Models in a Realistic Deployment Scenario, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18363, https://doi.org/10.5194/egusphere-egu24-18363, 2024.